Pixel circuit and display device

ABSTRACT

A pixel circuit includes a switching transistor whose conduction is controlled by a drive signal supplied to the control terminal, a drive wiring adapted to propagate the drive signal, and a data wiring adapted to propagate a data signal. A multi-layered wiring structure is used so that a second wiring layer is formed on a layer different from that on which a first wiring layer is formed.

CROSS REFERENCES TO RELATED APPLICATIONS

This is a Continuation Application of U.S. patent application Ser. No.15/336,366, filed Oct. 27, 2016, which is a Continuation Application ofU.S. patent application Ser. No. 15/086,845, filed Mar. 31, 2016, nowU.S. Pat. No. 9,483,980, issued Nov. 1, 2016, which is a ContinuationApplication of U.S. patent application Ser. No. 14/556,508, filed Dec.1, 2014, now U.S. Pat. No. 9,324,738, issued Apr. 26, 2016, which is aContinuation Application of U.S. patent application Ser. No. 14/230,540,filed Mar. 31, 2014, now U.S. Pat. No. 8,994,623, issued Mar. 31, 2015,which is a Continuation Application of U.S. patent application Ser. No.13/137,707, filed Sep. 6, 2011, now U.S. Pat. No. 8,730,134, issued May20, 2014, which is a Divisional Application of U.S. patent applicationSer. No. 12/010,675, filed Jan. 29, 2008, now U.S. Pat. No. 8,013,812,issued Sep. 6, 2011, which in turn claims priority from JapaneseApplication No.:2007-033509, filed with the Japan Patent Office on Feb.14, 2007, the entire contents of which being incorporated herein byreference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a display device such as organic EL(Electro Luminescence) display having pixel circuits arranged in amatrix form, each of which has an electro-optical element whosebrightness is controlled by a current. The invention relatesparticularly to a so-called active matrix display device in which thecurrent flowing through an electro-optical element is controlled by aninsulated gate field effect transistor disposed in each pixel circuit.

2. Description of the Related Art

An image display device such as liquid crystal displays an image bycontrolling the optical intensity of each pixel according to imageinformation to be displayed. This is also true for organic EL and otherdisplays. However, organic EL display is a so-called spontaneousluminescent display having a light-emitting element in each pixelcircuit. This type of display offers advantages including high imagevisibility, no need for backlights and high response speed.

Further, organic EL display differ significantly from liquid crystaldisplay and other types of display in that the brightness of eachlight-emitting element is controlled by a current flowing therethroughto provide color gradation. That is, the light-emitting elements arecurrent-controlled.

As with liquid crystal displays, organic EL displays can be driven bysimple or active matrix. It should be noted, however, that although theformers are simple in structure, they have disadvantages includingdifficulties in implementing a large-size, high-definition display. As aresult, the development of active matrix displays has been going on at abrisk pace in recent years. In this type of display, the current flowingthrough the electro-optical element in each pixel circuit is controlledby an active element, which is generally a TFT (Thin Film Transistor),provided in the same pixel circuit.

FIG. 1 is a block diagram illustrating the configuration of a typicalorganic EL display device.

A display device 1 includes a pixel array section 2 having pixelcircuits (PXLCs) 2 a arranged in an m by n matrix. The display device 1further includes a horizontal selector (HSEL) 3, write scanner (WSCN) 4,data wirings DTL1 to DTLn and scan lines WSL1 to WSLm. The data wiringsDTL1 to DTLn are selected by the horizontal selector 3 and supplied witha data signal commensurate with brightness information. The scan linesWSL1 to WSLm are selectively driven by the write scanner 4.

It should be noted that the horizontal selector 3 and write scanner 4may be formed on a polycrystalline silicon or around pixels using, forexample, a MOSIC.

FIG. 2 is a circuit diagram illustrating a configuration example of thepixel circuit 2 a in FIG. 1 (refer to, for example, U.S. Pat. No.5,684,365 and Japanese Patent Laid-Open No. Hei 8-234683).

The pixel circuit 2 a in FIG. 2, which is the simplest in configurationof a large number of circuits proposed, is a so-called dual-transistordrive circuit.

The pixel circuit 2 a in FIG. 2 includes p-channel thin film fieldeffect transistors (hereinafter referred to as “TFTs”) 11 and 12, acapacitor C11, and an organic EL element (OLED) 13 which is alight-emitting element. In FIG. 2, DTL and WSL represent the data wiringand scan line, respectively.

An organic EL element has often rectifying capability. As a result, itis sometimes referred to as an OLED (Organic Light Emitting Diode).Although represented by a diode symbol in FIG. 2 and other drawings, theorganic EL element is not necessarily demanded to offer rectifyingcapability in the description given below.

In FIG. 2, the TFT 11 has its source connected to a supply potentialVCC. The light-emitting element 13 has its cathode connected to a groundpotential GND. The pixel circuit 2 a in FIG. 2 operates in the mannerdescribed below.

Step ST1:

The scan line WSL is placed into a selected state (pulled down to lowlevel in this case). Then, a write potential Vdata is applied to thedata wiring DTL. As a result, the TFT 12 conducts, charging ordischarging the capacitor C11 and bringing the gate potential of the TFT11 to Vdata.

Step ST2:

The scan line WSL is placed into an unselected state (pulled up to highlevel in this case). This causes the data wiring DTL and TFT 11 to beelectrically isolated from each other. However, the gate potential ofthe TFT 11 is maintained constant by the capacitor C11.

Step ST3:

The current flowing through the TFT 11 and light-emitting element 13takes on a value commensurate with a gate-to-source voltage Vgs of theTFT 11. As a result, the light-emitting element 13 continues to emitlight at the brightness commensurate with the current.

The operation adapted to select the scan line WSL and convey thebrightness information, which has been given to the data wiring, to thepixel circuit is hereinafter referred to as “writing.”

As described above, the light-emitting element 13 in the pixel circuit 2a shown in FIG. 2 continues to emit light at a constant brightness oncethe potential Vdata is written. The light-emitting element 13 continuesto do so until the potential Vdata is rewritten.

As described above, the pixel circuit 2 a controls the current valueflowing through the light-emitting element 13 by changing the voltageapplied to the gate of the TFT 11 which serves as a drive transistor.

At this time, the p-channel drive transistor has its source connected tothe supply potential VCC. As a result, this TFT 11 operates in thesaturated region at all times. Therefore, the TFT 11 serves as aconstant current source whose current has the value shown in Equation 1below.

(Equation 1)

Ids=½*μ(W/L)Cox(Vgs−|Vth|)²  (1)

Here, μ is the carrier mobility, Cox the gate capacitance per unit area,W the gate width, L the gate length, Vgs the gate-to-source voltage ofthe TFT 11, and Vth the threshold of the TFT 11.

With a simple matrix image display device, each light-emitting elementemits light only instantaneously when selected. In contrast, with anactive matrix display device, the light-emitting elements continue toemit light even after the writing is complete, as described above. As aresult, an active matrix display device can provide high peak brightnessand reduced peak current as compared to a simple matrix display device,making this type of display device advantageous particularly when it isused in a large-size, high-definition display.

FIG. 3 is a view illustrating the secular change of the current vs.voltage (I-V) characteristic of the organic EL element. In FIG. 3, thecurve shown by a solid line represents the characteristic in the initialstate, whereas the curve shown by a dashed line represents thecharacteristic following a secular change.

The I-V characteristic of the organic EL element generally deterioratesover time as illustrated in FIG. 3.

However, the dual-transistor drive circuit shown in FIG. 2 is driven bya constant current. As a result, a constant current continues to flowthrough the organic EL element. This keeps the organic EL element freefrom secular deterioration of emission brightness even in the event of adeterioration of the I-V characteristic thereof.

Incidentally, the pixel circuit 2 a in FIG. 2 includes p-channel TFTs.However, if the same circuit 2 a includes n-channel TFTs, the existingamorphous silicon (a-Si) process can be used to manufacture TFTs. Thisprovides reduced costs for TFT substrates.

Next, a basic pixel circuit will be described in which p-channel TFTsare replaced by n-channel TFTs.

FIG. 4 is a circuit diagram illustrating a pixel circuit which includesn-channel TFTs in place of p-channel TFTs shown in FIG. 2.

A pixel circuit 2 b shown in FIG. 4 includes n-channel TFTs 21 and 22, acapacitor C21, and an organic EL element (OLED) 23 which is alight-emitting element. In FIG. 4, DTL and WSL represent the data wiringand scan line, respectively.

In the pixel circuit 2 b, the TFT 21 serves as a drive transistor. TheTFT 21 has its drain connected to the supply potential VCC and itssource connected to the anode of the EL element 23, thus forming asource follower circuit.

FIG. 5 is a view illustrating an operating point of the TFT 21, whichserves as a drive transistor, and the EL element 23 in the initialstate. In FIG. 5, a drain-to-source voltage Vds of the TFT 21 is plottedalong the horizontal axis, and a drain-to-source current Ids thereofalong the vertical axis.

As illustrated in FIG. 5, the source voltage is determined by theoperating point of the TFT 21, which serves as a drive transistor, andthe EL element 23. This voltage varies depending on the gate voltage.

The TFT 21 is driven in the saturated region. As a result, the currentIds shown in Equation 1 flows through the TFT 21. The current Ids isrelated to Vgs which is associated with the source voltage at theoperating point.

SUMMARY OF THE INVENTION

The pixel circuit described above is the simplest of all circuits.Practically, however, the circuit contains additional components such asa TFT connected in series with the organic EL element to serve a drivetransistor and other TFTs adapted to cancel the mobility and threshold.

For these TFTs, a gate pulse is generated by vertical scanners providedon both or one side of the active matrix organic EL display panel. Thispulse signal is transmitted through wirings and applied to a desired TFTin the pixel circuits arranged in a matrix form.

If this pulse signal is applied to two or more TFTs, the timings atwhich the signal is applied are important.

As illustrated in FIG. 6, however, a drive wiring which applies thepulse signal to the gate of the transistor (TFT) in the pixel circuit 2a via a buffer 40 is generally made of Mo (molybdenum). A wiringresistance r gives rise to a pulse delay and transient changes. Thisleads to a time lag, causing shading and banding. The greater thedistance from the scanner, the larger the resistance of the wiringleading to the transistor gate in the pixel circuit 2 a.

This leads to a difference in mobility correction period, for example,between the two ends of the panel, resulting in a brightness difference.

Further, because of a deviation of the mobility correction period fromthe optimal one, the mobility correction fails to correct the variationof the mobility of some pixels, thus resulting in stripes viewable onthe screen.

There is a need for the present invention to provide a pixel circuit anddisplay device using the same capable of suppressing shading and bandingresulting from the resistance of a wiring through which a gate pulse issupplied.

A pixel circuit according to a first embodiment of the present inventionincludes at least one switching transistor whose conduction iscontrolled by a drive signal supplied to the control terminal, a drivewiring adapted to propagate the drive signal, and a data wiring adaptedto propagate a data signal. The drive wiring is formed on a first wiringlayer and connected to the control terminal of the switching transistor.The data wiring is formed on a second wiring layer and connected to afirst terminal of the switching transistor. A multi-layered wiringstructure is used so that the second wiring layer is formed on a layerdifferent from that on which the first wiring layer is formed.

Preferably, the drive wiring layer is formed with the same material asthe data wiring layer.

Preferably, the drive wiring layer is formed with Al, aluminum.

A display device according to a second embodiment of the presentinvention includes a plurality of pixel circuits arranged in a matrixform. Each of the plurality of pixel circuits includes at least oneswitching transistor whose conduction is controlled by a drive signalsupplied to the control terminal, a drive wiring adapted to propagatethe drive signal, a data wiring adapted to propagate a data signal, andan electro-optical element whose brightness changes with change incurrent flowing. The display device further includes a first scanneradapted to output the drive signal onto the drive wiring and a secondscanner adapted to output the data signal onto the data wiring. Thedrive wiring is formed on a first wiring layer. The drive wiring isconnected to the control terminal of the switching transistor and thefirst scanner. The data wiring layer is formed on a second wiring layer.The data wiring layer is connected to a first terminal of the switchingtransistor and the second scanner. A multi-layered wiring structure isused so that the second wiring layer is formed on a layer different fromthat on which the first wiring layer is formed.

Preferably, the drive wiring layer is formed with the same material asthe data wiring layer.

Preferably, the drive wiring layer is formed with Al, aluminum.

According to the present embodiment, a pixel circuit includes at leastone switching transistor, drive wiring and data wiring. The pixelcircuit has a multi-layered wiring structure so that the drive wiring isformed on a first wiring layer, and the data wiring on a second wiringlayer.

The present embodiment can suppress shading and banding resulting fromthe resistance of a wiring through which a gate pulse is supplied.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the configuration of a typicalorganic EL display device;

FIG. 2 is a circuit diagram illustrating a configuration example of apixel circuit shown in FIG. 1;

FIG. 3 is a view illustrating the secular change of the current vs.voltage (I-V) characteristic of an organic EL element;

FIG. 4 is a circuit diagram illustrating a pixel circuit which includesn-channel TFTs in place of p-channel TFTs shown in FIG. 2;

FIG. 5 is a view illustrating an operating point of the TFT 21, whichserves as a drive transistor, and the EL element 23 in the initialstate;

FIG. 6 is a view for describing disadvantages caused by wiringresistance;

FIG. 7 is a block diagram illustrating the configuration of an organicEL display device using pixel circuits according to an embodiment of thepresent invention;

FIG. 8 is a circuit diagram illustrating the concrete configuration ofthe pixel circuit according to the present embodiment;

FIG. 9 is a view for describing a first example of remedy againstshading and banding;

FIGS. 10A and 10B are views for describing a short circuit caused by awiring pattern;

FIG. 11 is a view illustrating a configuration example using amulti-layered wiring structure; and

FIGS. 12A to 12F are timing diagrams for describing the operation of thepresent embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will be described below withreference to the accompanying drawings.

FIG. 7 is a block diagram illustrating the configuration of an organicEL display device using pixel circuits according to an embodiment of thepresent invention.

FIG. 8 is a circuit diagram illustrating the concrete configuration ofthe pixel circuit according to the present embodiment.

A display device 100 includes a pixel array section 102 having pixelcircuits 101 arranged in an m by n matrix. The display device 100further includes a horizontal selector (HSEL) 103, write scanner (WSCN)104, drive scanner (DSCN) 105, and first and second auto-zero circuits(AZRD1) 106 and (AZRD2) 107. The display device 100 still furtherincludes data wirings DTL, scan line WSL and drive line DSL. The datawirings DTL are selected by the horizontal selector 103 and suppliedwith a data signal commensurate with brightness information. The scanline WSL is selected and driven by the write scanner 104 and serves as afirst drive wiring. The drive line DSL is selected and driven by thedrive scanner 105 and serves as a second drive wiring. The displaydevice 100 still further includes first and second auto-zero lines AZL1and AZL2. The first and second auto-zero lines AZL1 and AZL2 areselected and driven respectively by the first and second auto-zerocircuits (AZRD1) 106 and 107.

The pixel circuit 101 according to the present embodiment includes ap-channel TFT 111, n-channel TFTs 112 to 115, a capacitor C111,light-emitting element 116 which includes an organic EL element (OLED:electro-optical element), and first and second nodes ND111 and ND112.

A first switching transistor is formed by the TFT 114, second switchingtransistor by the TFT 113, third switching transistor by the TFT 115,and fourth switching transistor by the TFT 111.

It should be noted that the supply line of the supply voltage VCC(supply potential) corresponds to a first reference potential, and theground potential GND to a second reference potential. Further, VSS1corresponds to a fourth reference potential, and VSS2 to a thirdreference potential.

In the pixel circuit 101, the TFT 111, TFT 112 serving as a drivetransistor, first node ND111 and light emitting element (OLED) 116 areconnected in series between the first reference potential (supplypotential VCC in the present embodiment) and the second referencepotential (ground potential GND in the present embodiment). Morespecifically, the light emitting element 116 has its cathode connectedto the ground potential GND and its anode connected to the first nodeND111. The TFT 112 has its source connected to the first node ND111. TheTFT 111 has its drain connected to the drain of the TFT 112 and itssource connected to the supply potential VCC.

The TFT 112 has its gate connected to the second node ND 112. The TFT111 has its gate connected to the drive line DSL.

The TFT 113 has its drain connected to the first node 111 and the firstelectrode of the C111. The TFT 113 has its source connected to a fixedpotential VSS2 and its gate connected to the second auto-zero line AZL2.The capacitor C111 has its second electrode connected to the second nodeND112.

The TFT 114 has its source and drain connected between the data wiringDTL and second node ND112. The TFT 114 has its gate connected to thescan line WSL.

Further, the TFT 115 has its source and drain connected between thesecond node ND112 and predetermined potential Vss1. The TFT 115 has itsgate connected to the first auto-zero line AZL1.

As described above, in the pixel circuit 101 according to the presentembodiment, the capacitor C111 is connected as a pixel capacitancebetween the gate and source of the TFT 112 serving as a drivetransistor. The source potential of the TFT 112 is connected to thefixed potential via the TFT 113 serving as a switching transistor duringa non-emission period. Further, the gate and drain of the TFT 112 areconnected together, thus allowing for correction of the threshold Vth.

In the display device 100 according to the present embodiment, amaterial having a resistance lower than that of molybdenum is used forthe gate wiring leading from the final stage (output stage) of thevertical scanner to the gate of the TFT (transistor) in the pixelcircuit 101. Molybdenum is typically used for this purpose. Thisprevents shading and banding caused by a pulse delay resulting from theresistance of the wiring through which a drive pulse is supplied to theTFT gate in the pixel circuit 101.

This remedy against shading and banding is applied at least to the scanline WSL among the wirings, namely, the scan line WSL, drive line DSLand first and second auto-zero lines AZL1 and AZL2.

A first example of remedy will be described below. In this description,a case will be shown in which the remedy is applied to the scan lineWSL.

FIG. 9 is a view for describing an example of remedy against shading andbanding.

In FIG. 9, 1041 denotes a final stage (output stage) buffer of the writescanner 104. This buffer is formed as a CMOS buffer for PMOS and NMOStransistors PT1 and NT1. On the other hand, the gate of the TFT 114serving as a switching transistor of the pixel circuit 101 is connectedto the final stage of the write scanner 104. A drive wiring 200 of thescan line WSL has a resistance r′.

In the present example, aluminum is used for the scan line WSL (drivewiring 200) and data wiring DTL. The wiring resistance r′ of the scanline WSL made of aluminum is lower than the resistance r of the sameline WSL made of molybdenum. The resistance r′ is about one tenth theresistance r.

As described above, aluminum is used for both the scan line WSL and datawiring DTL in the present example of remedy, thus suppressing pulsesignal delay and transient changes.

Incidentally, molybdenum is typically used for the scan line WSL, andaluminum for the data wiring DTL. These wirings and the pixel circuit101 are laid out on a semiconductor substrate. However, using the samematerial, namely, aluminum, for the scan line WSL and data wiring DTL asin the present example results in a short circuit at an intersectingpoint of the scan line WSL and data wiring DTL.

This short circuit in the wirings will be described with reference toFIGS. 10A and 10B.

FIGS. 10A and 10B are views for describing a short circuit caused by awiring pattern.

FIG. 10A illustrates the structure around a gate section 114 a of theTFT 114 (refer to FIG. 9) when molybdenum is used for the scan line WSL(drive wiring 200), and aluminum for the data wiring DTL. On the otherhand, FIG. 10B illustrates the structure around the gate section 114 aof the TFT 114 when aluminum is used for both the scan line WSL and datawiring DTL. In FIGS. 10A and 10B, 201 denotes an Al/Mo(aluminum/molybdenum) contact, and 202 an Al/Poly (aluminum/polysilicon)contact.

As illustrated in FIG. 10A, the scan line WSL is formed with molybdenum,and the data wiring DTL with aluminum. As a result, no short circuitoccurs at an intersecting point of the scan line WSL and data wiringDTL. However, if both the scan line WSL and data wiring DTL are formedwith aluminum as illustrated in FIG. 10B, a short circuit occurs at anintersecting point of the two.

To avoid a short circuit at an intersecting point of the scan line WSLand data wiring DTL, the present example employs a multi-layered wiringstructure for the scan line WSL and data wiring DTL.

This multi-layered wiring structure will be described with reference toFIG. 11.

FIG. 11 is a view illustrating a configuration example using amulti-layered wiring structure.

As illustrated in FIG. 11, the data wiring DTL is brought up to a newlayer 301 using a material such as TiAl. As a result, in thisconfiguration, the new layer 301 is provided on the second wiring layerwhich is higher in level than the first wiring layer on which the scanline WSL (drive wiring 200) is provided. Further, an Al/Al (new layer)contact 302 is provided on the new layer 301 in this configuration. Thenew layer 301 is connected to the first terminal of the TFT 114 via thecontact 302. Still further, the scan line WSL is connected to the gatesection 114 a of the TFT 114 in this configuration. In the presentexample of remedy, the data wiring DTL transmits data signals using thenew layer 301, and the drive wiring transmits drive signals.

It should be noted that both the scan line WSL and new layer 301 aremade of the same material, namely, aluminum. In this case, the typicalTFT process can be used.

As described above, a multi-layered wiring structure makes it possibleto avoid a short circuit resulting from crossing of the wirings evenwhen the scan line WSL and data wiring DTL are both made of aluminum.

The operation of the above configuration will be described next withreference to FIGS. 12A to 12F with emphasis on the operation of thepixel circuit.

FIG. 12A illustrates a drive signal DS applied to the drive line DSL,FIG. 12B a drive signal WS applied to the scan line WSL, FIG. 12C adrive signal AZ1 applied to the first auto-zero line AZL1, FIG. 12D adrive signal AZ2 applied to the second auto-zero line AZL2, FIG. 12E thepotential of the second node ND 112, and FIG. 12F the potential of thefirst node ND 111.

The drive signal applied to the drive line DSL by the drive scanner 105is maintained at high level. The drive signal WS applied to the scanline WSL by the write scanner 104 is maintained at low level. The drivesignal AZ1 applied to the first auto-zero line AZL1 by the firstauto-zero circuit 106 is maintained at low level. The drive signal AZ2applied to the second auto-zero line AZL2 by the second auto-zerocircuit 107 is maintained at high level.

As a result, the TFT 113 turns on, causing a current to flow through theTFT 113. This brings a source potential Vs of the TFT 112 (potential ofthe node ND 111) down to VSS2. As a result, the voltage applied to thelight-emitting element 116 becomes zero, causing the same element 116 tostop emitting light.

In this case, even if the TFT 114 turns on, the voltage held by thecapacitor C111, namely, the gate voltage of the TFT 112, remainsunchanged.

Next, during a non-emission period of the EL light-emitting element 116,the drive signal AZ1 applied to the first auto-zero line AZL1 is pulledup to high level while the drive signal AZ2 applied to the secondauto-zero line AZL2 is maintained at high level. This brings thepotential of the second node ND 112 down to VSS1.

Then, after the drive signal AZ2 applied to the second auto-zero lineAZL2 is switched back to low level, the drive signal DS applied to thedrive line DSL by the drive scanner 105 is switched back to low levelonly for a predetermined period of time.

This causes the TFT 113 to turn off and the TFTs 115 and 112 to turn on.As a result, a current flows through the TFTs 112 and 111, raising thepotential of the first node ND 111.

Then, the drive signal DS applied to the drive line DSL by the drivescanner 105 is switched to high level, and the drive signal AZ1 to lowlevel.

As a result, the threshold Vth of the drive transistor TFT 112 iscorrected, bringing the potential difference between the second andfirst nodes ND 112 and ND 111 to Vth.

This condition is maintained for a predetermined period of time, afterwhich the drive signal WS applied to the scan line WSL by the writescanner 104 is maintained at high level. Data is written to the node ND112 from the data wiring DTL. While the drive signal WS is at highlevel, the drive signal DS applied to the drive line DSL by the drivescanner 105 is switched to low level. Then, the drive signal WS isswitched to low level after a while.

At this time, the TFT 112 turns on, and the TFT 114 turns off, allowingthe mobility to be corrected.

In this case, the TFT 114 is off. The gate-to-source voltage of the TFT112 is constant. As a result, the constant current Ids flows from theTFT 112 into the EL light emitting element 116. This raises thepotential of the first node ND 111 to a voltage Vx where the current Idsflows through the EL light emitting element 116, causing the sameelement 116 to emit light.

Also in the present circuit, the current vs. voltage (I-V)characteristic of the EL element changes if the light emission timethereof is long. This causes the potential of the first node ND 111 tochange as well. However, the gate-to-source voltage Vgs of the TFT 112is maintained constant. As a result, the current flowing through the ELlight emitting element 116 remains unchanged. Hence, even if the I-Vcharacteristic of the same element 116 deteriorates, the constantcurrent Ids continues to flow. As a result, the brightness of the sameelement 116 remains unchanged.

When the pixel circuit has a multi-layered wiring structure in which thescan line (drive wiring) and data wiring (new layer) are formed withaluminum as in the present example of remedy, the remedy is applied tothe entire panel to prevent shading and banding resulting from a drivesignal (pulse) delay due to wiring resistance. This ensures high qualityimage with minimal shading and banding.

A second example of remedy will be described next. In the presentexample, a multi-layered wiring structure is used as in the firstexample. The scan line WSL (drive wiring 200) is formed with Ag(silver), and the data wiring DTL with aluminum.

A wiring resistance r″ of the scan line WSL made of silver is lower thanthe resistance r of the same line WSL made of molybdenum. Thissuppresses pulse signal delay and transient changes, thus providing thesame effect as with the first example of remedy.

Further, the present embodiment provides the same effect as in the firstand second examples of remedy when a multi-layered wiring structure isused as in the first and second examples and when a material lower inresistance than aluminum is used for the new layer 301. For example,silver is used for the new layer 301.

This ensures reduced impact of wiring resistance on the signalpropagation, thus providing high quality image with minimal shading andbanding.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

What is claimed is:
 1. A display device having a multi-layer wiringstructure, comprising: a plurality of pixel circuits located in adisplay area; a first wiring extending through the display area andconfigured to propagate a control voltage; and a second wiring extendingthrough the display area and configured to propagate a predeterminedvoltage, wherein at least one of the plurality of pixel circuitsincludes: a light emitting element; a switching transistor including acontrol terminal, a channel portion, a first current terminal and asecond terminal, the control terminal electrically connected to thefirst wiring, and the first current terminal electrically connected tothe second wiring; a driving transistor including a channel portion, acontrol terminal, a first current terminal and a second terminal, thecontrol terminal of the driving transistor electrically connected to thesecond current terminal of the switching transistor, and the firstcurrent terminal of the driving transistor electrically connected to ananode electrode of the light emitting element; a capacitive elementincluding a first electrode and a second electrode, the first electrodeelectrically connected to the second terminal of the switchingtransistor so as to receive a voltage dependent on the predeterminedvoltage via the switching transistor, and the second electrodeelectrically connected to the first current terminal of the drivingtransistor; and wherein the first wiring is a portion of a first layer,the second wiring is a portion of a second layer which is different fromthe first layer, and a first intermediate wiring is a portion of a thirdlayer which is different from a fourth layer, the channel portion, thefirst current terminal and the second current terminal of the switchingtransistor being portions of the fourth layer, the first intermediatewiring being electrically connected between the second current terminalof the switching transistor and the first electrode of the capacitiveelement, and wherein an electrical path from the switching transistor tothe capacitive element is formed by employing an semiconductor-aluminumcontact and an aluminum-molybdenum contact.
 2. The display deviceaccording to claim 1, wherein the first wiring and the second wiringinclude a same material.
 3. The display device according to claim 2,wherein the first wiring and the second wiring include aluminum.
 4. Thedisplay device according to claim 1, wherein the second wiring includessilver and the first intermediate wiring includes aluminum.
 5. Thedisplay device according to claim 1, wherein the first current terminal,the channel portion and the second current terminal of the switchingtransistor are continuously formed on a fourth layer, and the thirdlayer is disposed on the fourth layer.
 6. The display device accordingto claim 1, wherein the channel portion of the switching transistorincludes poly-silicon.
 7. The display device according to claim 1,wherein the control terminal of the switching transistor includes aplurality of gate electrodes continuously formed with the first wiring.8. The display device according to claim 7, wherein at least one of theplurality of gate electrodes is a protruding portion of the firstwiring.
 9. The display device according to claim 1, wherein the firstwiring corresponds to the scan line.
 10. The display device according toclaim 1, wherein the second wiring corresponds to the signal line, thepredetermined voltage corresponds to an image voltage data, and thelight-emitting element is configured to emit light in response to avalue of the image voltage data.
 11. The display device according toclaim 1, wherein the switching transistor includes a lower resistancematerial than molybdenum.
 12. The display device according to claim 1,wherein the first wiring is electrically connected to the first currentterminal of the switching transistor via a second intermediate wiring.